1. Field of the Invention
The present invention relates to an f..theta. lens system for use in laser printers and the like, and more particularly to an f..theta. lens system with variable focal length.
2. Description of the Related Arts
There is known a laser printer in which a photosensitive medium is scanned by laser beam modulated according to image data. The modulated laser beam is deflected by a deflector such as a polygon mirror to scan the beam and is transmitted to the photosensitive medium by a projection lens system. Such the projection lens system should have a characteristic of distortion described by y=f..theta., wherein y is image height, f is focal length of the lens system and .theta. is beam incident angle to the lens system, so that the beam is moved and scanned on the photosensitive medium while maintaining the moving speed thereof constant. The projection lens system may also be called the f..theta. lens system. Practically, it is required for the f..theta. lens system to have small difference between actual distortion and the equation y=f..theta., small astigmatism and flat image plane as possible.
The conventional f..theta. lens system has been designed to have the fixed focal length, so that a magnification ratio of the projected image is kept constant. The magnification ratio has been changed by electrical modification of image data rather than by using an optical arrangement with variable magnification property. However, the electrical modification of image data is likely to result in damage of partial image data.
In the case of different size prints, the conventional laser printer has been provided with a circuitry for controlling a start timing of beam modulation and an end timing thereof by line according to a paper size to be used for printing. The control circuitry was more complex and more expensive.